Ferromagnetic garnet with stress insensitive b-h loop and method of making same

ABSTRACT

YTTRIUM IRON GARNET FERRITES, WHEREIN THE INTRODUCTION OF CERIUM OXIDE INTO THE GARNET REDUCES THE MECHANICAL PRESSURE STRESS DEPENDENCE OF REMANENT MAGNETIZATION AND COERCIVE FORCE, ARE DISCLOSED.

' Feb. 16, 1971 R.QG.WEST

RNET WITH STRESS IN FERROMAGNETC GA SENSITIV B-H LOOP AND METHOD OFMAKING SAME 2 Sheets-$heet 1 Filed April 5, 195a- EFFECT OF COMPRESSIVESTRESS ON B OF Ce CONTAINING JYIG m T. O o .I e O SC I IO 0 S O O O O O2 e P l ..w m w w w 3 O C ,o/v C G 0 m W 3 m N 3 i O o O T l I O O m C Mm. 0 l\ 2 FR 2 2 8 To E U '0 e U 0 S 2 A C M 2 O a m ,0, E w R o m -w Pl C m J O Ov K w I C 3 0| w. w 5 5 LOP 5 O 5 5 O m 2 O 7 5 Y. L L l. n O0 0 2 22mm Dmrrmmm wUmOu MZUIMOU Pussez. z. 74 25 7" ATTORNEY 5COMPRESSIVE STRESS ON v Feb. 16,1971 R. G.WEST 3,563,897

' FERROMAGNETC GARNET WITH STRESS INSENSITIVE B-H LOOP AND METHOD OFMAKING SAME Filed April 5, 1968 I 2 Sheets-$heet 2 OF 3 e 5 |2 EFFECT OFMECHANICAL THE HYSTERESIS LOOP OF 2.925 Ce 0.075 5|2 EFFECT OFMECHANICAL COMPRESSIVE STRESS ON THE HYSTERESIS LOOP 0F 2.s5 e o.|5 es 0I2 INVENTOR Faisal; G PI Esr ATTORNEYS United States PatentFERROMAGNETIC GARNET WITH STRESS IN- SENSITIVE B-I-I LOOP AND METHOD OFMAKING SAME Russell G. West, Alexandria, Va. Trans-Tech, Inc., 12 MeemAve., Gaithersburg, Md. 20760) Filed Apr. 3, 1968, Ser. No. 718,536 Int.C1. C04!) 35/40 U.S. Cl. 25262.57 11 Claims ABSTRACT OF THE DISCLOSUREYttrium iron garnet ferrites, wherein the introduction of cerium oxideinto the garnet reduces the mechanical pressure stress dependence ofremanent magnetization and coercive force, are disclosed.

BACKGROUND OF THE INVENTION Ferromagnetic garnet materials have beenwidely used, especially in microwave devices, since their discovery. Ithas, however, been noted that the yttrium iron garnet does notadequately satisfy certain design requirements for optimum microwavedevices.

Yttrium iron garnet ferrites (hereinafter YIG ferrites), will produce arotation of a plane of polarization of an incident plane polarizedelectromagnetic wave when properly disposed in a wave guide and biasedby a magnetic field. The direction of the biasing magnetic fielddetermines the manner in which the high frequency energy is affected.This property of ferrite materials has resulted in the development ofnonreciprocal devices which utilize this rotation, commonly known as theFaraday rotation.

The prior art has substituted certain other ions in the YIG structurefor certain magnetic effects. For instance, the Geller U.S. Pat. No.3,006,855 discloses the use of garnet compounds such as calcium-tingarnets used in solution with YIG ferrites or other rare earth irongarnets, to produce ferrites with magnetic moments sub stantially higherthan that of pure rare earth iron garnet. The Harrison et al. U.S. Pat.No. 3,132,105 discloses the incorporation of gadolinium in YIG ferritesto produce a relatively low saturation magnetization value,

which remains substantially constant over an extended,

range of temperature values. A similar disclosure is found in theZneimer et al. Pat. No. 3,125,534. The Geller Pat. No. 3,006,854discloses incorporation of zirconium or hafnium ions into YIG ferritestructures.

One of the problems encountered by the user of YIG ferrites in microwaveapplications relates to stress-induced changes of magnetization.Mechanical stresses induce a variation of the remanent magnetization ofYIG ferrites. That is, tensile or compressive stresses will alter theremanent magnetization of YIG ferrites, as well as altering the coerciveforce and the shape of the B-H hysteresis loop, of the ferrites. Thiseffect is especially noticeable when a ferromagnetic toroid is utilizedin microwave digital phase shifters. The toroid is rigidly located in awave guide, and any change in tensile or compressive stresses, such as,for instance, those caused by temperature changes, will result in analteration of the differential phase, since the phase shift is directlyrelated to the remanence magnetization magnitude. This magnetizationdependency upon stress is described by Stern and Temme, IEEE Trans. MTI,vol. 3, p. 873 (1965).

Comstock and Raymond, Journal of Applied Physics, vol. 38, No. 9, p.3,737 (August 1967) attempted to determine theoretical magnetostrictiondata for ytterbium and cerium iron garnets. The authors measured thetemperature dependence of various constants of Yb in YIG and of Ce inYIG. The cerium iron garnet values "ice were determined by testingferrites made by doping single crystals of YIG with cerium ions. Thesemeasurements allowed the authors to make theoretical predictions of thetemperature dependence of certain magnetostriction constants of pureytterbium iron garnet and pure cerium iron garnets (which cannot begrown per se).

Dillon and Neilsen Physical Review, vol. 120, No. 1, pp. -113 (Oct. 1,1960) performed resonance experiments on single crystals of YIG doped ina melt with rare earth ions, including Ce+++. The authors found nosignificant effects in the cerium doping.

Blasse and Bril, Applied Physics Letters," vol. II, No. 2, pp. 5355(July 15, 1967) added Ce O to Y O and hydrated alumina to produce Y Al OCe garnets. Chemical analysis excluded other oxidation states of ceriumthan Ce The garnet showed a bright yellow emission under excitation withcathode rays as well as with blue radiation, suggesting the use thereoffor phosphors for flying-spot cathode-ray tubes for color television (YAl O is known to be non-magnetic).

SUMMARY OF THE INVENTION Yttrium iron garnet ferrites with stressinsensitive B-H hysteresis loops are produced by incorporating ceriumoxide into the oxide mixture prior to the firing of oxides to producethe garnet ferromagnetic material. Generally, from about 0.5 to about7.5 mole percent of the cerium oxide, based on the moles of total oxide,will be utilized. The remanent magnetization of the resultant ferritesis relatively insensitive to tensile and compressive stresses, and theferrites are particularly suitable for use in microwave differentialphase shift devices.

DESCRIPTION OF THE INVENTION The introduction of oxides of cerium intoyttrium iron garnet has an appreciable effect in controlling the stressdependence of the remanent magnetization of the ferrite. In addition,the coercive force of the YIG ferrite can be made stress-insensitive forquite large variations in mechanical stress. The microwave properties ofthe resultant ferrite are not appreciably deteriorated. To date, singlecrystals of YIG have been only of academic interest as toroids inmicrowave digital applications. It will therefore be appreciated thatthe present invention is concerned only with polycrystalline garnetstructures, wherein the structure is of reduced stress dependency of BHloops.

The microwave garnets of the present invention can be produced byconventional processes. In one process, the carefully weighed rawmaterials are first wet mixed with water, in a steel ball mill whichpreferably has a rubber or plastic lining. Other blending apparatus,such as high-speed blenders, colloid mills and attritors may be used.The resultant homogeneous mixture is then oven dried or spray dried. Inthis drying operation, water should be removed quite rapidly in order tominimize preferential settling of denser or larger particles. Ifdesired, the mixture may be pressure filtered before drying to expeditewater removal.

The dried material may then be presintered, or calcined, by heattreating at a temperature somewhat lower than the final firingtemperature. This presintering step generally helps control shrinkage inthe final stage and improves homogeneity. All or any portion of thematerial may be presintered as desired, as known to the art. After thepresintering step, the material is comminuted to a particle size that isceramically workable, preferably about two microns or less in size.

Several means of forming the comminuted particles into the desiredshapes may be utilized, including die pressing, extruding andhydrostatic or isostatic pressing.

Organic additives may be introduced at this point to serve as a binderand particle lubricant, as desired.

The ferromagnetic ferrites are produced by a solid state reaction attemperatures of 1100 to 1500 C. If an organic additive is used, theformed shapes must be heated gradually during the low temperature rangein the firing cycle in order to slowly volatize the organic additive.(Rapid evolution of vapors may crack the formed shapes.)

Hot pressing, which involves the simultaneous pressing and firing of theferrite, may be used to advantage in many instances. The concurrentapplication of heat and pressure generally produces good control ofdensity and crystal grain size.

It is known that ferrite materials are generally susceptible to changesin oxygen content during firing. It is therefore preferred to controlthe chemical valence of the compounds by precise firing schedules andcontrolled atmospheres during the heat treatment, as known to the art.

The YIG ferrites can be produced by the methods disclosed in U.S. Pats.Nos. 3,131,082, 3,062,746 or 3,051,- 656, the disclosures of which arehereby incorporated by reference.

Any one or several of the rare earths of atomic number 62-71 may besubstituted for a portion of the yttrium in the ferrite of the presentinvention. Of these, gadolinium is the preferred material. Also, theiron may be replaced in part by aluminum, gallium, or indium, of whichaluminum is the most preferred. Such replacements or substitutions willbe by way of additions of the respective oxides (or precursors) in lieuof the replaced or substituted oxide. As used herein, unless otherwisespecified, the term yttrium iron garnet is to be construed to meanyttrium with and without the presence of up to about 40 weight percentof the garnet of oxides of the rare earths of atomic number 6271 andwith and without the additional presence of oxides of aluminum, galliumor indium as partial substitution (up to 11 weight percent of thegarnet) for the iron.

The garnet structure of the present invention will therefore berepresented by the following composition:

wherein Z is yttrium alone or mixed with minor amounts (up to about 40weight percent of the garnet) of oxides (or precursors) of rare earthsof atomic numbers 62 to 71 inclusive, A is Fe or Fe mixed with up toabout 11 weight percent of the garnet of oxides (or precursors) of Al,Ga or In, and x is a positive number of 0.02 to 0.30. Preferably, Z ispure yttrium or yttrium with up to about 38.5% by weight of the garnetof gadolinium oxide. A is preferably pure Fe or Fe mixed with up toabout 10.5% by weight of the garnet of A1 0 x is preferably 0.05 to 0.15and most preferably about 0.08.

Formula I It is to be emphasized that the above formula does notnecessarily represent a given compound, but is only indicative of theelements in the garnet composition.

The cerium preferably is added as CeO or a precursor thereof, such as acarbonate, acetate, oxalate, hydroxide and the like, or mixturesthereof. For instance, heating dry cerous carbonate at about 570 C.produces ceric oxide. Kirk-Othmer, Encyclopedia of Chemical Technology"2nd Edition, vol. 4 pp. 840-852, the disclosure of which is herebyincorporated by reference, sets forth properties of a number of ceriumcompounds,

CeO is of quite high melting point (2600 C.) which is well above thegarnet firing temperatures. The oxidation state of the cerium in thegarnet structure is not known. It is possible that the Ce converts, atleast in part, to Ce at the sintering temperatures. However, it is to beemphasized that the nature of such a conversion, if any, is not known atthis time. The experimental results, as set forth in the workingexamples below, suggest that the mechanical pressure stress dependencyof the remanent magnetization and the coercive force of YIG garnetferrites could be reduced by the addition of Ce O or a precursor of Ce Ointo the oxide starting material mixture. However, the Handbook ofChemistry and Physics, Chemical Rubber Publishing Co., 36th Edition,indicates that cerous oxide (Ce O ignites at about 200 0, suggestingthat an inert atmosphere would be required. It will therefore beappreciated that the preferred embodiment of the present inventionutilizes cerium oxide (CeO or precursors thereof.

Regardess of the mechanism by which the cerium oxide is incorporatedinto the ferrite material, the presence of minor amounts of oxides ofcerium render the BH hysteresis loop stress insensitive, or relativelystress insensitive, which results in reducing the stress dependence ofthe remanent magnetization (remanent induction) and coercive forces, aswell as illustrating a squarer B-H hysteresis loop under stress. Theaddition of cerium oxide also improves the high power handlingcharacteristics of the garnet.

The preferred ferrites of the present invention can be represented bythe following atomic composition:

wherein v is 0 to about 2.0, x is 0.02. to 0.30, preferably 0.05 to0.15, and most preferably 0.08, and n is 0 to about 1.5.

The microwave properties of cerium-containing YIG ferrites are notappreciably deteriorated. The 9.4 gHz. dielectric loss tangent generallyremains below about 0.0001. The saturization magnetization appears to berelatively insensitive to cerium oxide in the YIG ferrite. The spin waveline width increases rapidly, and the ferromagnetic resonance line widthincreases moderately, with increasing levels of cerium oxide.

The present invention will be more clearly understood from theaccompanying drawings, wherein FIG. 1 is a diagram illustrating theeffect of compressive stress upon the remanent induction (remanentmagnetization) of YIG ferrites;

FIG. 2 is a diagram illustrating the effect of compressive stress uponthe coercive force of YIG ferrites;

FIG. 3 is a B-H hysteresis loop for pure unmodified YIG;

FIG. 4 is a B-H hysteresis loop for modified YIG,

' wherein x=0.075; and

FIG. 5 is a B-H hysteresis loop for modified YIG wherein x=0.l5.

The methods used to determine the values shown in FIGS. 1 and 2 aredescribed in the Trans-Tech, Inc. advertisement entitled Test forHysteresis Loop Properties appearing in Microwave, Hayden MicrowaveCorp., New York, N.Y., vol. 5, No. 9, p. 39 (Sept. 1966).

A controlled mechanical compressive force was applied into the majoraxis of the toroid under test. The cerium containing garnets wereproduced according to the examples of this application (the pure YIGgarnet was produced in the same manner as Example 3, with the exceptionthat no CeO was utilized in the mixture, with compensating addition of YO All ferrites tested were of the same identical size and shape. 1.86mole percent CeO represents the situation wherein x in Formula 1 is0.075 and 3.68 mole percent CeO represents the situation wherein x inFormula 1 is 0.15. It will be readily seen that the compositioncontaining 1.68 mole percent CeO is relatively stress insensitivebetween 0 and 3000 p.s.i. compressive stress, especially when comparedto the pure YIG ferrite material. As will be seen from FIG. 2,increasing amounts of cerium oxide, up to about 3.68 mole percent,render the resultant YIG ferrite more and more stress insensitive incoercive force. (It will be noted that at 3.68 mole percent CeO thecurve is essentially flat.)

However, it should be noted in FIG. 1 that the composition containing3.68 mole was not as stress insensitive in remanent magnetization as thegarnet containing 1.68 mole percent CeO A comparison of FIGS. 3, 4 and 5will show the remarkable improvement in the reduction of stresssensitivity of yttrium iron garnets accomplished by the presentinvention. The B-H loops of FIGS. 4 and 5 are much squarer under stressand less stress sensitive than the B-H loop of FIG. 3. The compressivestress value of 3000 p.s.i. was arbitrarily chosen for the above tests.The reduction in stress sensitivity obtained by the cerium-containinggarnets of the present invention will be noted for much lower and muchhigher pressures. For instance, the slopes of the curves of FIGS. 1 and2 indicate that pure YIG ferrite, unlike the compositions of the presentinvention, is quite stress sensitive at low pressures.

Generally, amounts of cerium oxide in the YIG ferrite corresponding tovalues of x in Formula 1 of less than 0.02 will not have an appreciableeffect upon the ferromagnetic properties of the resultant garnet.Amounts of cerium oxide greater than that represented by the value of0.30 for x in Formula 1 will result in a garnet which is of increasingstress dependency in remanent induction and coercive force, as comparedto cerium oxide levels within the ranges herein.

The invention will be understood more readily by reference to thefollowing examples; however, these examples are intended to illustratethe invention and are not to be construed to limit the scope of theinvention.

EXAMPLES OF THE INVENTION Example 1 (Preparation of modified YIG garnetof the composition 2.925 0.0'l5 5 12') The following materials weremixed together:

Grams Y O (1.50 micron average particle size) 44.487 Ce (1.65 micronaverage particle size) 1.739 FegO (0.5 micron average particle size)53.774

(All particle sizes were measured on a Fisher Sub-Sieve Sizer.)

The oxides were blended together in a Waring Blender -with 150 ml. ofdistilled water for 15 minutes. The resultant slurry was placed in acovered steel pan and heated at 200 C. for about 20 minutes to evaporatethe water. The dried cake was then calcined in air at 1250 C. for 8hours. The calcined powder was placed in a 500 ml. polyethylene jarcontaining a A volume charge of in. stainless steel balls in 120 ml. ofdistilled water and milled for 10 hours, at which time the averageparticle size was less than 2 microns. The resultant milled slurry wasdried in a covered steel pan at 200 C. for about 20 minutes. The driedpowder was granulated by forcing it through a No. 20 sieve, and then wasisostatically pressed into compacts at a pressure of 6000 p.s.i.g. Thecompacts used in all of the examples were toroids with rectangular outerdimensions of 1 /2" x 1" x A centrally located rectangular hole 1" x A3"passed through the compact,

opening on the 1 /2" x 1" sides. The compacts were sintered in oxygen(air) at a temperature of 1480 C. for 10 hours. The sintered ferriteswere machined to outer dimensions of 1% x x The centrally located holewas not machined, but a volume shrinkage of about 17% was noted. Thecompressive stresses were applied to the x sides of the toroid in allB-H load determinations.

The garnet had the atomic composition:

2-925 ao'z5 12 and had the following properties:

41rM (gauss) 1750 e 14.4 tan 6, dielectric loss tangent 0.0001 AH(oersteds) 130 g-Elfective 2.00 AH (oersteds) 5.2

The B-H hysteresis loop for the garnet is shown in FIG. 4 of theaccompanying drawings.

Example 2 (Preparation of modified YIG garnet of the composi- Thefollowing materials were mixed together:

Grams Y O (1.50 micron average particle size) 44.393 CeO (1.65 micronaverage particle size) 1.854 Fe O (0.5 micron average particle size)53.753

The garnet of this example was prepared using the same procedure asdescribed in Example 1.

The resultant garnet structure corresponded to the following atomiccomposition: Y Ce Fe O The garnet had the following intrinsic electricaland magnetic properties:

tan 6, dielectric loss tangent 0.0001 6 14.7 AH (oersteds) 99 AH(oersteds) 5.8 g-Etfective 1.99 411-M (gauss) 1600 The B-H hysteresisloop of the garnet produced by this example was very similar to thatshown in FIG. 4.

Example 3 (Preparation of modified YIG garnet of the compositiOn Y2 5ce5Fe5O 2J The following oxides were mixed together:

Grams Y O (1.5 micron average particle size) 43.088 CeO (1.65 micronaverage particle size) 3.457 Fe O (0.5 micron average particle size)53.455

The garnet of this example was prepared using the procedure of Example1, and the garnet had the atomic composition Y Ce -Fe O The garnet hadthe following properties:

41rM (gauss) 1760 e n 14.2 tan 6, dielectric loss tangent 0.0001 AH(oersteds) g-Effective 2.03 AH (oersteds) 8.6

The BH hysteresis loop for this garnet is shown in FIG. 5 of theaccompanying drawings.

Example 4 (Preparation of modified YIG garnet of the composition Y Ce FeO The following oxides were used as starting materials:

While the coercive force of this garnet was of reduced compressivestress sensitivity, the remanent magnetization was much more sensitiveto compressive stress than the garnet of Example 3. This suggests thatthis composition is about the upper limit of CeO addition for the moststress-insensitive B-H loop garnets.

The resultant garnet corresponded to the following atomic composition:

Example 5 (Preparation of modified YIG garnet of the composition Y2 9gC002F65012-) A modified YIG garnet was produced utilizing the followingraw materials:

Grams Y O (1.50 micron average particle size) 45.523 CeO (1.65 micronaverage particle size) 0.466 Fe O (0.5 micron average particle size)54.011

The garnet of this example was prepared using the same procedure asdescribed in Example 1. The resultant garnet had the following atomiccomposition:

The garnet produced by this example had a BH loop which was squarerunder stress and less sensitive to compressive stresses than the samegarnet (of FIG. 3) without the cerium oxide present. The improvement instress insensitivity Was marginal. suggesting that this example may betaken as the general lower working limit of cerium oxide content.

Example 6 (This example relates to a cerium and gadolinium modified YIGgarnet of the composition The oxides used Were:

Grams Y O (1.5 micron average particle size) 14.608 CeO (1.65 micronaverage particle size) 1.591 Gd O (0.6 micron average particle size)37.684 Fe O (0.5 micron average particle size) 46.117

The garnet was produced using the procedure of Example l.

The resultant garnet structure corresponded to the following atomiccomposition: Y Ce Gd Fe O The garnet produced by this example had a BHloop which was squarer under stress and less sensitive to compressivestresses than the same garnet without the cerium oxide present(compensated by the stoichiometric addition of Y O of course).

Example 7 (This example relates to a cerium and aluminum modified YIGgarnet of the composition asz aoa a.e 1.4 12) The oxides used were:

Grams Y O (1.5 micron average particle size) 49.949 Ce0 (1.65 micronaverage particle size) 1.961 Fe O (0.5 micron average particle size)40.930 A1 0 (1.35 micron average particle size) 10.160

Example 8 (This example relates to the preparation of a cerium,gadolinium and aluminum modified YIG garnet of the COlnpOSltiOn Y2 52Ce0ggGd0 FC4 Al0 4O 8 The oxides used were:

Grams Y O (1.5 micron average particle size) 37.515 CeO (1.65 micronaverage particle size) 1.815 Gd O (0.6 micron average particle size)9.558

Fe O (0.5 micron average particle size) 48.424 A1 0 (1.35 micron averageparticle size) 2.688

The garnet was produced by the procedure of Example 1, except that thecompact was sintered in oxygen (air) at a temperature of 1495 C. for 10hours.

The resultant garnet structure corresponded to the following atomiccomposition:

The garnet produced by this example had a BH loop which was squarerunder stress and less sensitive to compressive stresses than the samegarnet Without the cerium oxide present (compensated by thestoichiometric addition of Y O of course).

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. Polycrystalline yttrium iron garnet structure of reduced mechanicalstress dependency of remanent magnetization and coercive forcesconsisting essentially of a modified yttrium iron garnet represented bythe atomic composition:

wherein Z is selected from the group consisting of yttrium and mixturesthereof with at least one rare earth of atomic numbers 62-71 inclusivethe amount of rare earth, calculated as oxide being up to about 40% byweight of the garnet A is selected from the group consisting of iron andmixtures thereof with at least one member selected from the groupconsisting of aluminum, gallium and indium, the amount of aluminum,gallium and indium, calculated as oxide being up to about 11% by weightof the garnet, and x is about 0.02 to about 0.30.

2. A composition as claimed in claim 1 wherein x is 0.05 to 0.15.

3. A composition as claimed in claim 1 wherein x is about 0.08.

4. The composition of claim 1 wherein said modified yttrium iron garnetis represented by the atomic composition:

wherein v is 0 to about 2.0, x is about 0.02 to about 0.30, and n is 0to about 1.5.

5. A composition as claimed in claim 4 wherein v and n are both 0.

6. A composition as claimed in claim 5 wherein x is 0.05 to 0.15.

7. A composition as claimed in claim 5 wherein x is about 0.08.

8. A composition as claimed in claim 1 wherein said rare earth isgadolinium present in an amount, calculated as oxide of up to about38.5% by weight of the garnet.

9. A composition as claimed in claim 1 wherein A is iron with up to10.5% by weight of the garnet of aluminum, calculated as oxide.

10. In a process for producing polycrystalline yttrium iron garnetferrites of the Z A O type wherein Z is selected from the groupconsisting of yttrium and mixtures thereof with at least one rare earthof atomic numbers 6271 inclusive, the amount of rare earth, calculatedas oxide being up to about 40% by weight of the garnet, and A isselected from the group consisting of iron and mixtures thereof with atleast one member selected from the group consisting of aluminum, galliumand indium,

the amount of aluminum, gallium and indium, calculated as oxide being upto about 11% by weight of the garnet, said process comprising the stepsof mixing and blending of raw materials of said yttrium iron garnetferrites together in the form of oxides or oxide precursors, forming theblended material into formed shapes, and thereafter sintering saidformed shapes at temperatures of up to about 1500" C. to obtainferromagnetic garnets, the improvement comprising adding cerium as anoxide or precursor thereof, as a substitute for a portion of theyttrium, in an amount of from 0.5 to 7.5 mole percent, calculated asoxide, to said raw materials, based on the total moles of raw materials.

11. The process as claimed in claim 10 wherein about number of moles ofsaid raw materials, are added.

References Cited UNITED STATES PATENTS 4/1963 Gorter et al 252-62.57

OTHER REFERENCES TOBIAS E. LEVOW, Primary Examiner J. COOPER, AssistantExaminer US. Cl. X.R. 252-6258

